1. Field of the Invention
The present invention relates to a pulse signal delay circuit which can be applied in an LED drive circuit and the like and generates a delay pulse signal by copying an input pulse signal with a delay of a predetermined delay time, as well as an LED drive circuit using the same.
2. Description of the Related Art
In an LED drive circuit and the like using a DC/DC converter technology, a smoothing condenser is charged by a booster circuit via Schottky diode and a constant voltage value is set. A current is supplied to a load connected parallel to the smoothing condenser from the booster circuit and its smoothing condenser. Recently, as the size of devices has been reduced, the use of a ceramic condenser as a smoothing condenser has increased. In some cases, ceramic condensers having a relatively large capacitance used as a smoothing condenser emit abnormal sounds due to vibrations caused by the piezoelectricity of the dielectric material used in the condenser as a voltage is applied. According to the prior art, such abnormal sounds have been prevented by changing the circuit constant or the direction in which the condenser is mounted.
Meanwhile, backlights for displays in portable apparatuses such as cellular phones have been converted to LEDs. In particular, in a case where an IC for an LED driver having the booster circuit as described above and a smoothing condenser using ceramics are combined in an apparatus which is used in close proximity to the ear, such as a cellular phone, abnormal sounds as described above cannot be ignored. In particular, in cellular phones, the sound quality is made important such that it is becoming inevitable to prevent the emission of abnormal sounds from the apparatus. Against this background, it has been proposed to make the pulse width modulated signal (PWM signal) for driving the LED circuit be 20 kHz or higher, as disclosed in Japanese Unexamined Patent Publication 2006-114324, as a measure for preventing abnormal sounds.
However, the following problems arise with the method for setting the frequency of the PWM signal high as disclosed in the Japanese Unexamined Patent Publication 2006-114324. First, the range of the frequency of the PWM signal is limited, and thus, the freedom in setting the frequency is decreased and the freedom in designing is decreased. Second, in the case of driving an LED circuit in which LEDs are aligned in an array by aligning a number of LED series circuits in parallel where a number of LEDs are connected in series in the same direction all the LED series circuits are turned on and off simultaneously at a high frequency, and therefore, the load is suddenly changed, and thus, the ceramic condenser used in the constant voltage power supply vibrates due to the piezoelectricity, thereby causing sounds. That is, a further measure is required to prevent the emission of abnormal sounds in LED circuits where LEDs are aligned in an array.
The second problem will now be described with reference to FIGS. 8 and 9. FIG. 8 shows an example of a circuit configuration of an LED circuit 60 and a booster circuit 70 for supplying a current in order to drive the respective LEDs in the LED circuit 60. The LED circuit 60 is formed by connecting a number (n) of circuit units 64 (641 to 64n) in parallel, which are formed by connecting an LED series circuit 62, where a number of LEDs 61 are connected in series in the same direction, and a drive transistor 63 in series. The booster circuit 70 is a DC/DC converter circuit which switches the input current supplied from an external power supply voltage Vcc via a coil 71 through a switching transistor 72 made up of an N channel type MOSFET and a resistor 73, and thus, boosts the power supply voltage Vcc and charges a smoothing condenser 75 with the boosted voltage via Schottky barrier diode 74 so that the voltage is smoothed. The switching transistor 72 is turned on and off when a switching control circuit 78 monitors the voltage gained by dividing the output voltage Vout smoothed by the smoothing condenser 75 by resistors 76 and 77, and the output voltage Vout is controlled so as to have a desired voltage value. In an example of the circuit configuration in FIG. 8, the output voltage Vout from the booster circuit 70 is supplied to the anode side of each LED series circuit 62 in the LED circuit 60. A pulse width modulated signal (PWM signal) Sin for driving the LED circuit 60 is supplied to the gate of the drive transistor 63 in each circuit unit 64 from the outside as a gate signal which is inputted to all of the gates.
When the frequency of the PWM signal Sin is set to a certain frequency or higher (for example, 200 Hz or higher), the brightness is leveled for the human eye. Therefore, each circuit unit 64 is controlled so that the same amount of current flows through the LEDs 61, and it becomes possible to adjust the brightness of the entirety of the LED circuit 60 by changing the duty ratio of the PWM signal Sin.
When the drive transistor 63 in each circuit unit 64 is controlled so as to be turned on and off according to the same timing, however, the LED drive current which flows through each circuit unit 64 changes greatly according to the timing of the switching for turning on and off the drive transistor 63, and therefore, as shown in FIG. 9, the output voltage Vout fluctuates by approximately several volts. This fluctuation of the output voltage Vout at the time of switching is described with reference to FIG. 9.
When the PWM signal Sin drops, the drive transistor 63 is turned off. As a result, the supply of the current to each LED series circuit 62 is stopped, the current load of the booster circuit 70 is reduced, and the output voltage Vout of the booster circuit 70 is increased to a set voltage or higher. In addition, the output of current is blocked on the LED circuit 60 side, and therefore, it becomes unnecessary for the booster circuit 70 to supply a current to the LED circuit 60, being a load circuit, and the booster circuit 70 stops the operation when the voltage of the smoothing condenser 75 returns to the set value.
Next, when the PWM signal Sin rises, the drive transistor 63 is turned on, and the supply of a current to each LED series circuit 62 suddenly starts, and therefore, the current load in the booster circuit 70 becomes heavier. As a result, a current is supplied from the smoothing condenser 75 in response to a sudden increase in the current load, and therefore, the output voltage Vout of the smoothing condenser 75 lowers. As the output voltage Vout lowers, the booster circuit 70 starts the boosting operation, and the voltage of the smoothing condenser 75 lowers before the boosting operation starts. After the booster circuit 70 starts the boosting operation, the boosting operation is controlled so that the voltage of the smoothing condenser 75 is recovered to the set value, and thereby recovering the set value.
However, a sudden fluctuation in the output voltage Vout when the PWM signal Sin rises and drops generates vibrations due to the piezoelectricity of the smoothing condenser 75, thereby causing unnecessary sounds.
It is herein considered that the amplitude of the sudden fluctuation of the output voltage Vout is restricted in order to prevent the smoothing condenser 75 from vibrating. As one measure in this case, it is considered to drive the drive transistors 63 in a number of circuit units 64 with a time difference by shifting the drive time. As shown in FIG. 10, for example, the PWM signal Sin is directly inputted into the gate of the drive transistor 63 in one circuit unit 641 and delays signals Sd2 to Sdn gained by delaying the PWM signal Sin in sequence using delay circuits 79 (for example, inverter column circuits) of which the number is the same as the number (n−1) of other circuit units 642 to 64n, and thus, it is considered to input the signals to the gates of the drive transistors 63 in the circuit units 642 to 64n, respectively.
However, the following problem arises with this measure. That is, the rising edge delay time for the input pulse signal in the delay circuit 79 is not necessarily equal to the falling edge delay time, and therefore, the duty ratio of the PWM signal Sin changes whenever the PWM signal passes through the delay circuit 79. As a result, the brightness becomes inconsistent between the circuit units 64, and this causes flickering throughout the entirety of the LED circuit 60.